This is a difficult question to answer with a single number, as the accuracy of a machine is dependent upon multiple factors, including the machine size and design, the components used, and the time and care taken during assembly.
That being said, we can make some generalizations based on our own tests and on data from our customers. In general, accuracy of +/-0.005" can be achieved on cuts without too much trouble in a localized area. Repeatability is typically better, and on well-tuned systems, repeatability of 0.002" or better has been reported.
One specific area of concern related to accuracy is backlash. Backlash is positioning error caused when an axis switches direction. If the direction of rotation changes, any lag in the change of direction in linear motion is not seen by the system and results in positioning error. On both our R&P drives and Acme screw based systems, properly setup systems have very low amounts of backlash, typically less than 0.001" in our PRO setups. To put that in perspective, that's about the thickness of 1/4 of a sheet of notebook paper.
One of the most important design aspects of a machine that impacts accuracy is the height of the z axis. The z is the one cantilevered axis in a machine, so deflection and vibration are highest here. Limiting the length of the lever arm of the z is by far the best way to achieve superior accuracy. For light materials like foam and balsa, higher z axes are possible, but for wood or aluminum, minimizing z height is important.